One year from today, a tiny black dot will creep across the face of the Sun: the planet Venus. It'll take more than six hours to pass from one edge of the Sun to the other, so at least some of the event -- known as a transit -- will be visible from most of Earth.

The transit will be fun to watch, but it won't have much scientific value. That wasn't always the case, though. Transits of Venus were some of the most widely anticipated scientific events of the 18th and 19th centuries.

The first transit ever recorded took place in 1639. Around that time, astronomers realized that you could use transits to measure the distance from Earth to Venus, which in turn would reveal the distance to the Sun and all the other bodies of the solar system; more about that tomorrow.

But it was a long wait for a chance to try it out. Transits occur in pairs, with a gap of eight years between the first transit and the second. But the pairs are separated by more than a century. There were two transits in the 18th century, two in the 19th, and none in the 20th. So after the transit of 1639, the next one didn't come along until 1761.

Using the transits of the 18th and 19th centuries, astronomers were able to calculate the Earth-Sun distance to within a fraction of a percent of the true distance -- a bit less than 93 million miles.

So mark your calendars for next June's transit of Venus -- the last one until the year 2117.

Script by Damond Benningfield, Copyright 2011

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